Emergency power units which supply electrical or hydraulic power for critical flight control systems on an aircraft, following a loss of main engine power, must come on line and begin producing power virtually instantaneously when they are needed. The same is true for power transmission or generation apparatus used in stationary emergency power generators which supply electricity to a computer or a hospital, for instance, in the event of a power outage in a municipal power grid. These systems must provide power virtually instantaneously despite the fact that they may have been sitting idle for extended periods of time at ambient temperatures of -20.degree. F. or below.
Such emergency power units often utilize a hydraulic clutch to couple an engine or other prime mover to a generator, a pump, a gearbox, or some other driven device. The clutch allows the driven device to be de-coupled from the engine during startup to reduce the load on the engine start system. The clutch also allows the engine to be rapidly decoupled from the driven device for safety or other reasons. In some instances the clutch is further required to provide a controlled amount of slip to achieve a smooth engagement, or to compensate for rapid fluctuations in the load imposed on the engine by the driven device.
A hydraulically actuated clutch, as well as the control system for engaging the clutch, must be properly designed to allow rapid engagement of the clutch after prolonged exposure to low ambient temperatures. In general, the clutch and control system must either include features that compensate for the increased viscosity of hydraulic fluids at low temperatures, or some means must be provided for heating the cold viscous fluid, to lower its viscosity, prior to engaging the clutch. If such compensation or heating features are not provided, unstable clutch operation of the clutch may occur during an attempt to engage the clutch at low temperatures.
In the past, a number of different approaches have been utilized to provide temperature compensation or pre-heating of the hydraulic fluid. Perhaps the simplest approach is to allow the hydraulic fluid to circulate within the control circuit and warm-up for some period of time prior to attempting an engagement of the clutch. The circulating fluid is heated by friction and dynamic pressure losses within the hydraulic circuit. While this approach has the advantage of being extremely simple to implement, since no additional compensation or heating devices are required, it can result in severe operating penalties due to the long periods of time involved to warm the fluid to an acceptable operating temperature.
Alternatively, various types of heating devices have been utilized to more rapidly raise the temperature of the fluid. In some prior applications, electrical resistance heaters, engine exhaust gas, or engine coolant circuits have been utilized to either maintain the fluid in a ready-state at operating temperature, or in an on-demand mode to rapidly heat the fluid to operating temperature following a command to engage the clutch. For preheating devices other than the "ready-state" type, some delay is still encountered, but the clutch can usually be engaged significantly faster than in prior applications relying solely on circulation of the fluid to cause heating by fluid resistance and dynamic pressure loss. In other prior clutch control systems, temperature compensating features are included in control valves, etc., to accommodate changes in fluid viscosity which could otherwise result in unstable operation at low temperatures.
In addition to dealing with control stability problems, some drive systems must also deal with high viscous drag torques, upon start-up at cold temperatures, in gearboxes or other driven devices connected to the output of the drive system. These viscous drag forces are the result of cold viscous lubricant in bearings, gears, seals, etc. of the driven device. In some prior drive systems where such high viscous drag is present, the clutch itself has been oversized to prevent burn-out. The addition of temperature compensation for achieving control stability and over-sizing the clutch, as described above, have obvious disadvantages of increasing system complexity, size, weight and cost.
In yet another prior approach, some drive systems utilize a fluid coupling operably joining the prime mover to a driven device in a parallel drive arrangement with the hydraulically actuated clutch. The fluid coupling is utilized to provide smoother engagement of the hydraulically actuated clutch over a range of operating temperatures and speeds. Specifically, during start-up at low temperatures the fluid coupling is engaged first, and drives the output speed up to approximately match input speed. The hydraulically actuated clutch is then engaged, and the fluid coupling is disengaged. The fluid coupling allows a limited amount of slip between the input and output speeds during start-up. This limited amount of slip compensates for the higher start torque at low temperatures caused by cold viscous fluid in the clutch control system, or in bearings, gears, etc., of the drive mechanism. By not requiring the hydraulically actuated clutch to engage until input and output speeds are approximately equal, the danger of burn-out of the hydraulically actuated clutch is significantly reduced, U.S. Pat. No. 3,247,936 to Aschauer and 3,088,563 to Petrie illustrate drive devices of this type.
It is generally highly desirable to design the fluid coupling portion of such devices to have a power transmitting efficiency that is as high as possible to minimize the amount of time it takes for the fluid coupling to bring the output speed up to a speed at which the hydraulically actuated mechanical drive portion of the clutch can be safely engaged. Any inefficiency in the fluid coupling is primarily converted into heating of the fluid. While this heating due to coupling inefficiency does serve to help the fluid to more rapidly warm up, it is generally considered to be a relatively insignificant or even an undesirable side-effect to the primary goal of providing a high efficiency fluid coupling to allow rapid engagement of the mechanical drive portion of the drive apparatus.
To achieve high power transmitting efficiency the fluid coupling portion of combined fluid/mechanical driven systems, great care must be taken in the design and manufacture of the fluid coupling portion of the drive system. Complex shapes and close tolerances may be required for components within the fluid coupling. The need for such complex shapes to achieve high efficiency drives up the cost, weight, and size of prior fluid coupling/mechanical drive couplings to the point that they have not been a practical solution to the problem of providing rapid engagement of a hydraulically actuated clutch at low temperatures in many compact, low cost power plants, such as those described above.
Accordingly, it is an object of my invention to provide an improved hydraulically actuated drive apparatus, suitable for use in a wide range of power plants, that is capable of rapidly engaging after prolonged exposure to low ambient temperatures. It is also an object of my invention to provide an improved hydraulic clutch, and a control system for that clutch which is capable of providing rapid actuation and stable control of the clutch after prolonged exposure to low ambient temperatures. Additional objects of my invention include providing:
1. a gas turbine engine powered auxiliary power unit (APU) capable of providing a controlled power output in less than 30 seconds after receiving a start command when the APU has been exposed for an extended period of time to ambient temperatures of -20.degree. F.; PA1 2. a hydraulic control system capable of engaging and providing stable control of a hydraulic clutch in less than 30 seconds after prolonged exposure to ambient temperatures of -20.degree. F.; PA1 3. an on-demand heating system which is applicable to hydraulic systems having either single or multiple branches and capable of heating at least a portion of the fluid in the system to a temperature at which a control circuit of the hydraulic system can engage and achieve stable control of a fluid actuated device, such as a hydraulic clutch, in less than 30 seconds after prolonged exposure to ambient temperatures of -20.degree. F.; and PA1 4. a straightforward and inexpensive arrangement for producing a hydraulic system meeting the above stated objects.